I don't why he couldn't just go through with the original spell and then tell Ned and MJ again afterward. Would Strange have forgotten in that second too and wondered why there was some kid there? Seems like he would have bought it if Peter told him that he did it and specifically what he just did.
He hesitated at least partly because he thought MJ might only be with him because he was Spiderman
Also because he kind of just wanted everything to be perfect. Goblin even says it.